Modify transparency of a text object along the timeline using Python scripting
You need to use a material mat applied to each text object, and set its use_transparency property to true :   

mat.use_transparency = True
mat.transparency_method = 'Z_TRANSPARENCY'


Then, around the particular t you want the text to have full opacity, you need to set the aplha property of the material, and insert a keyframe corresponding to the alpha

for i in range(anim_length):
    alpha = (t - i) / anim_length + 1 - t / anim_length
    scene.frame_set(t - i)
    mat.alpha = alpha
    mat.keyframe_insert(data_path='alpha')
    scene.frame_set(t + i)
    mat.alpha = alpha
    mat.keyframe_insert(data_path='alpha')


Hope that helps.